The advent of Ocean Color remote sensing has provided oceanographers with the ability to observe synoptically the temporal and spatial variability of phytoplankton pigments on a global scale. Through the use of bio-optical algorithms, gross primary production estimates can be estimated from pigment biomass by knowing the chlorophyll-specific absorption coefficient, quantum yield and incident irradiance. Several recent studies have shown that cellular characteristics which ultimately affect the optical properties of phytoplankton vary predictably with temperature, light, and nutrient limitation. I propose to test the accuracy of a bio-optical model in the Greenland Sea during a mono-specific bloom of the prymnesiophyte, Phaeocystis. The model will be parameterized based on results from culture of Phaeocystis grown under temperature and light limitation uder nutrient-replete conditions. Model output will be compared to existing primary production data from the CEAREX (Coordi nated Eastern Arctic Experiment) cruise to the Greenland Sea. Refinements to the model will contribute to a better understanding of the parameterization of bio-optical models that will ultimately predict primary production of Phaeocystis blooms using data from Ocean Color Satellites. Actual subcellular absorption is accomplished in the chloroplast thylakoids and the length of the pathway of light is determined by the distribution of internal cellular organelles. In a previous study using transmission electron microscopy, results have shown that thylakoid stacking is related to photoadaptatiion and showed a near 2 fold change in thylakoid stacking when grown at 14 and 259 5mol quanta m-2S-1. I proposed to use electron microscope tomograms in order to examine how the internal organization of the single cells of Phaeocystis affects the a*ph (?? and ? under temperature limitation. The use of electron microscope tomograms will give us insight on subcellular organization of single cells of Phaeocystis which will help us to understand bulk absorption properties. Several tomographic reconstructions have been performed. These tomographic reconstructions will be used to examine the relationship between chloroplast structure and the absorption properties of Phae ocystis using Monte Carlo simulations.